Srs carrier based switching on unlicensed bands

ABSTRACT

Systems and methods related to switched carrier Sounding Reference Signal (SRS) transmission in unlicensed spectrum are disclosed. In some embodiments, a method of operation of a User Equipment device (UE) in a wireless system comprises performing uplink Listen-Before-Talk (LBT) on one or more candidate carriers for switched carrier SRS transmission and performing a switched carrier SRS transmission on at least one candidate carrier of the one or more candidate carriers that is determined to be available as a result of performing the uplink LBT on the one or more candidate carriers. In some embodiments, the one or more candidate carriers are carriers other than carriers configured for the UE and on which the UE is scheduled to transmit.

RELATED APPLICATIONS

This application is a continuation of U.S. patent application Ser. No.number 16/319,002, filed Jan. 18, 2019, which is a national stageapplication of International Patent Application No. PCT/IB2017/054400,filed Jul. 20, 2017, which claims the benefit of provisional patentapplication serial number 62/364,454, filed Jul. 20, 2016, thedisclosures of which are hereby incorporated herein by reference intheir entireties.

TECHNICAL FIELD

Sounding Reference Signal (SRS), SRS carrier based switching, Long TermEvolution (LTE) unlicensed carriers, License Assisted Access (LAA),MulteFire, Fifth Generation (5G) New Radio (NR) in unlicensed spectrum.

BACKGROUND

The ongoing standalone Long Term Evolution (LTE) in Unlicensed spectrumforum (MulteFire) and Third Generation Partnership Project (3GPP)Release (Rel) 14 work item on uplink License Assisted Access (LAA)allows LTE User Equipment devices (UEs) to transmit on the uplink in theunlicensed 5 gigahertz (GHz) or license-shared 3.5 GHz radio spectrum.These uplink transmissions generally need to perform Listen-Before-Talk(LBT) prior to accessing the channel. There is also an ongoing 3GPPRel-14 work item on Sounding Reference Signal (SRS) Carrier BasedSwitching (SCBS). How SCBS is implemented in LAA and MulteFire andunlicensed spectrum LTE in general is an open issue.

Today, the unlicensed 5 GHz spectrum is mainly used by equipmentimplementing the IEEE 802.11 Wireless Local Area Network (WLAN)standard, also known under its marketing brand as “Wi-Fi.”

LTE uses Orthogonal Frequency Division Multiplexing (OFDM) in thedownlink and Discrete Fourier Transform (DFT)-spread OFDM (also referredto as Single-Carrier Frequency Division Multiple Access (SC-FDMA)) inthe uplink. The basic LTE downlink physical resource can thus be seen asa time-frequency grid as illustrated in FIG. 1, where each ResourceElement (RE) corresponds to one OFDM subcarrier during one OFDM symbolinterval. The uplink subframe has the same subcarrier spacing as thedownlink and the same number of SC-FDMA symbols in the time domain asOFDM symbols in the downlink.

In the time domain, LTE downlink transmissions are organized into radioframes of 10 milliseconds (ms), each radio frame consisting of tenequally-sized subframes of length TSUBFRAME=1 ms as shown in FIG. 2.Each subframe comprises two slots of duration 0.5 ms each, and the slotnumbering within a frame ranges from 0 to 19. For normal cyclic prefix,one subframe consists of 14 OFDM symbols. The duration of each symbol isapproximately 71.4 microseconds (μs) (including cyclic prefix).

Furthermore, the resource allocation in LTE is typically described interms of Resource Blocks (RBs), where a RB corresponds to 12 contiguoussubcarriers in the frequency domain. RBs are numbered in the frequencydomain, starting with 0 from one end of the system bandwidth.

In LTE, the uplink transmissions are dynamically scheduled, i.e., ineach downlink subframe the base station transmits control informationabout which terminals should transmit data to the enhanced or evolvedNode B (eNB) in subsequent subframes, and upon which RBs the data istransmitted. The uplink resource grid is comprised of data and uplinkcontrol information in the Physical Uplink Shared Channel (PUSCH),uplink control information in the Physical Uplink Control Channel(PUCCH), and various reference signals such as Demodulation ReferenceSignals (DMRSs) and SRSs. An example uplink subframe is shown in FIG. 3.Note that uplink DMRS and SRS are time-multiplexed into the uplinksubframe, and SRSs are always transmitted in the last symbol of a normaluplink subframe. DMRSs are used for coherent demodulation of PUSCH andPUCCH data. The PUSCH DMRS is transmitted once every slot for subframeswith normal cyclic prefix, and is located in the fourth and eleventhSC-FDMA symbols.

The subframes in which SRSs are transmitted by any UE within a cell areindicated by cell-specific broadcast signaling. A 4-bit cell-specific‘srsSubframeConfiguration’ parameter indicates 15 possible sets ofsubframes in which SRS may be transmitted within each radio frame. Asnoted before, the SRS transmissions are always in the last SC-FDMAsymbol in the configured uplink subframes, and PUSCH transmission maynot be permitted on these symbols.

SRS is not associated with any data or control information but isgenerally used to estimate the uplink channel quality for purposes offrequency-selective scheduling. In order to serve this purpose, it isnecessary that SRSs from different UEs with different soundingbandwidths can overlap. As illustrated in FIG. 3, interleaved FrequencyDivision Multiple Access (FDMA) is used for SRS with a repetition factorof 2, which implies that in the configured SRS bandwidth, the SRS willbe mapped to every other subcarrier in a comb-like fashion. This allowsmultiple UEs to simultaneously transmit SRS without overlap. The SRSsequence spans at least four RBs, and the maximum allowed bandwidth ofone SRS is dependent on the uplink system bandwidth and thecell-specific parameter srs-BandwidthConfig, C_(SRS)∈{0,1, . . . ,7}.For example, for an uplink system bandwidth of 110 RBs and C_(SRS)=0,the maximum possible SRS bandwidth for a particular UE is 96 RBs. A2-comb (SRS on every other subcarrier) or 4-comb (SRS on every fourthsubcarrier) can be configured for SRS in Rel-13 LTE.

Furthermore, different phase or cyclic shifts can be applied to SRSsequences on the same REs to make them mutually orthogonal, with up toeight such UE-specific shifts currently available per comb. Thus, up to16 distinguishable full-bandwidth SRS sequences can currently beassigned to UEs. Up to six symbols of contiguous SRS transmission aresupported in Rel-13 LTE, for example in the Uplink Pilot Time Slot(UpPTS) region of a special subframe.

In Rel-14 LTE, a new 3GPP Work Item is ongoing regarding SRS carrierbased switching. The basic objective is as follows:

-   -   To support SRS switching to and between TDD [Time Division        Duplexing) component carrier(s), where the component carriers        available for SRS transmission correspond to the component        carriers available for carrier aggregation of PDSCH [Physical        Downlink Shared Channel], while the UE has fewer component        carriers available for carrier aggregation of PUSCH.

Therefore, UEs that are capable of uplink data transmission on a limitednumber of component carriers (e.g., two uplink component carriers can beused for PUSCH) can use SCBS to transmit SRS on many more ComponentCarriers (CCs). This is very beneficial since current UEs are limited toa maximum of two uplink CCs for PUSCH transmission, whereas the numberof downlink CCs can be as large as 32 in Rel-13 LTE. By configuringSCBS, the eNB can exploit downlink-uplink channel reciprocity andenhance downlink beamforming or scheduling on carriers for which the UEis unable to transmit PUSCH. However, this Work Item is mainly focusedon CCs in licensed spectrum. For SRS carrier based switching, the SRStransmission opportunities include:

-   -   In at least one symbol out of up to six symbols in a special        subframe    -   In the last symbol of an uplink subframe

Furthermore, both aperiodic and periodic SRS are supported for SCBS.

Up to now, the spectrum used by LTE is dedicated to LTE. This has theadvantage that the LTE system does not need to care about thecoexistence issue and the spectrum efficiency can be maximized. However,the spectrum allocated to LTE is limited which cannot meet the everincreasing demand for larger throughput from applications/services.Therefore, a new study item has been initiated in 3GPP on extending LTEto exploit unlicensed spectrum in addition to licensed spectrum.Unlicensed spectrum can, by definition, be simultaneously used bymultiple different technologies. Therefore, LTE needs to consider thecoexistence issue with other systems such as IEEE 802.11 (Wi-Fi).Operating LTE in the same manner in unlicensed spectrum as in licensedspectrum can seriously degrade the performance of Wi-Fi as Wi-Fi willnot transmit once it detects the channel is occupied.

Furthermore, one way to utilize the unlicensed spectrum reliably is totransmit essential control signals and channels on a licensed carrier.That is, as shown in FIG. 4, a UE is connected to a Primary Cell (PCell)in the licensed band and one or more Secondary Cells (SCells) in theunlicensed band. In this application an SCell in unlicensed spectrum isdenoted as an LAA SCell.

SRS in 3GPP Rel-14 enhanced LAA (eLAA) will be based on the legacy combdesign as in Rel-13 LTE. SRS when transmitted with PUSCH is located insymbol 13 of the uplink subframe. When SRS without PUSCH is transmittedin downlink ending partial subframes, the UE transmits SRS in symbol 13.The existing maximum number of SRS RBs is retained for a given systembandwidth in eLAA. No shifting of SRS is used on an LAA SCell. Onlyaperiodic SRS transmission is supported in eLAA. If the triggering forSRS without PUSCH is received in subframe n, the UE should send SRSwithout PUSCH in subframe n+k (not considering the LBT failure). Theoffset parameter k is indicated by three bits in the downlink grant,where “000” represents no triggering for SRS without PUSCH; “001”-“111”represents SRS without PUSCH is transmitted in subframe n+4 to n+11,respectively.

A new industry forum has been initiated on extending LTE to operateentirely on unlicensed spectrum in a standalone mode, which is referredto as “MulteFire” in marketing terms. There is no licensed carrier foressential control signal transmissions and control channels. Hence, thetransmission needs to be carried on the unlicensed spectrum with noguaranteed channel access availability and also fulfill the regulatoryrequirements on the unlicensed spectrum.

The use of a carrier in an unlicensed spectrum should be done in a fairand equal manner for different devices. One component when securing thisfair sharing is to have requirements on how to distribute transmissionsover the system bandwidth. Here, two requirements are commonly found inregulations:

-   -   1. Occupied channel bandwidth    -   2. Maximum Power Spectral Density (PSD)

For example, both these requirements are enforced for 5 GHz carriersaccording to ETSI 301 893, while only the maximum PSD requirements areenforced in the US regulation for 5 GHz.

The occupied bandwidth requirement is expressed as the bandwidthcontaining 99% of the power of the signal, shall be between 80% and 100%of the declared Nominal Channel Bandwidth. Our current understanding ofthis requirement is that it is tested over a time interval longer thanone subframe (1 ms). The frequency allocations for one UE must thus varybetween subframes in such a way that the requirement is fulfilled. It isstill an open issue if this requirement needs to be fulfilled for a UEwhich only transmits in a single subframe, such as Physical RandomAccess Channel (PRACH) or with a single PUSCH.

Maximum PSD requirements exist in many different regions. For most casesthe requirement is stated with a resolution bandwidth of 1 megahertz(MHz). For example, the ETSI 301 893 specification requires 10decibel-milliwatts (dBm)/MHz for 5150-5350 MHz. The implication of thePSD requirement on the physical layer design is that, without properdesigns, a signal with small transmission bandwidth will be limited intransmission power. This can negatively affect coverage of theoperation. That is, the maximum PSD requirement is a binding conditionthat requires changes to uplink transmissions in unlicensed spectrums.

3GPP has adopted interlaced transmissions as a means to give LAA uplinksignals with small bandwidth higher transmission powers when needed(and, to a lesser extent, to satisfy the transmission bandwidthrequirement). The interlacing of transmissions is done on a per PhysicalRB (PRB) basis. This design is also referred to as Block-InterleavedFDMA (B-IFDMA). Interlaced uplink transmissions are also used inMulteFire.

One interlace is illustrated in FIG. 5, in a design with five interlacesfor an example of 20 MHz system bandwidth with a maximum of 100 RBsavailable for transmission. As shown, a uniform spread of the RBs isconsidered, i.e., uniform interlaces where each interlace contains100/5=20 RBs. The figure to the right shows the first 1.2 MHz of thesame allocation. The hashed lines represent example boundaries of thePSD requirement measurement intervals (1 MHz resolution bandwidth). Theblack stripes represent the allocated RBs for the interlace.

Unlike 3GPP eLAA, the SRS transmissions in MulteFire are alsointerlaced. In other words, the SRS in MulteFire is different in thefrequency domain compared to eLAA, since the eLAA SRS follows the legacyLTE structure. An example of interlace-based SRS in MulteFire is shownin FIG. 6, where the SRS is located in symbol 13 in the time domain andon interlace #0 in the frequency domain.

In MulteFire SRS, comb structure is not supported. Therefore, cyclicshifts and/or Orthogonal Cover Code (OCC) are used to multiplexdifferent antenna ports from a UE, or to multiplex different UEs on thesame interlace. Up to four symbols of SRS can be transmitted by a UE inthe short PUCCH (sPUCCH) region of a partial downlink Transmit TimeInterval (TTI).

In LAA/standalone LTE Unlicensed (LTE-U) uplink, the SRS and SCBSoriginally designed for LTE on licensed spectrum cannot be reusedbecause of the following problems. First, on unlicensed carriers,channel access operates entirely based on the LBT mechanism. Unlikelicensed LTE, the channel access availability for SRSs transmitted onunlicensed carriers is not guaranteed. Second, the SRS multiplexingrules for UEs that are switching SRS to a particular carrier and UEsthat can send PUSCH on that carrier will be different for eLAA andMulteFire due to the different SRS structures.

SUMMARY

Systems and methods related to switched carrier Sounding ReferenceSignal (SRS) transmission in unlicensed spectrum are disclosed. In someembodiments, a method of operation of a User Equipment device (UE) in awireless system comprises performing uplink Listen-Before-Talk (LBT) onone or more candidate carriers for switched carrier SRS transmission andperforming a switched carrier SRS transmission on at least one candidatecarrier of the one or more candidate carriers that is determined to beavailable as a result of performing the uplink LBT on the one or morecandidate carriers. In some embodiments, the one or more candidatecarriers are carriers other than carriers configured for the UE and onwhich the UE is scheduled to transmit.

In some embodiments, performing the uplink LBT on the one or morecandidate carriers for switched carrier SRS transmission comprisesperforming the uplink LBT on a candidate carrier, a result of the uplinkLBT being that the candidate carrier is available, and performing theswitched carrier SRS transmission on the at least one candidate carriercomprises performing the switched carrier SRS transmission on thecandidate carrier in response to the result of the uplink LBT being thatthe candidate carrier is available. In some embodiments, the methodfurther comprises, prior to performing the uplink LBT on the candidatecarrier, starting uplink transmission on a first carrier, the firstcarrier being different than the candidate carrier. In some embodiments,the method further comprises, after performing the switched carrier SRStransmission on the candidate carrier, performing uplink LBT on thefirst carrier, a result of the uplink LBT on the first carrier beingthat the first carrier is available and resuming the uplink transmissionon the first carrier upon the result of the uplink LBT on the firstcarrier being that the first carrier is available.

In some embodiments, performing the uplink LBT on the one or morecandidate carriers for switched carrier SRS transmission comprisesperforming multi-carrier uplink LBT on a first set of carriers for whichthe UE is granted uplink transmission and a second set of carrierscomprising the one or more candidate carriers, the first set of carriersand the second set of carriers being disjoint sets, where the at leastone candidate carrier on which the UE performs switched carrier SRStransmission comprises at least one candidate carrier from the one ormore candidate carriers in the second set of carriers. Further, in someembodiments, performing multi-carrier uplink LBT on the first set ofcarriers for which the UE is granted uplink transmission and the secondset of carriers comprising the one or more candidate carriers comprisessuspending uplink transmission on at least one of the carriers in thefirst set of carriers prior to transmitting SRS on the at least onecandidate carrier. In some other embodiments, performing multi-carrieruplink LBT on the first set of carriers for which the UE is granteduplink transmission and the second set of carriers comprising the one ormore candidate carriers comprises suspending uplink transmission on allof the carriers in the first set of carriers prior to transmitting SRSon the at least one candidate carrier.

In some embodiments, performing the switched carrier SRS transmission onthe at least one candidate carrier comprises performing the switchedcarrier SRS transmission on the at least one candidate carrier inaccordance with an SRS transmission multiplexing configuration for arespective group of UEs such that the switched carrier SRS transmissionon the at least one candidate carrier is multiplexed with uplinktransmissions of another group of UEs on the same at least one candidatecarrier.

In some embodiments, performing the switched carrier SRS transmission onthe at least one candidate carrier comprises performing the switchedcarrier SRS transmission on the at least one candidate carrier inaccordance with an SRS transmission multiplexing configuration for arespective group of UEs such that the switched carrier SRS transmissionon the at least one candidate carrier is multiplexed with SRStransmission of another group of UEs on the same at least one candidatecarrier. Further, in some embodiments, the switched carrier SRStransmissions of the respective group of UEs are multiplexed with theSRS transmissions of another group of UEs in a particular symbol of thesame subframe on the same at least one carrier. In some otherembodiments, the switched carrier SRS transmissions of the respectivegroup of UEs are multiplexed with the SRS transmissions of another groupof UEs using different time-domain Orthogonal Cover Codes (OCCs). Insome other embodiments, the switched carrier SRS transmissions of therespective group of UEs are multiplexed with the SRS transmissions ofanother group of UEs using different time-domain OCCs and differentintra-symbol frequency-domain OCCs and cyclic shifts.

In some embodiments, a UE for a wireless system is adapted to performuplink LBT on one or more candidate carriers for switched carrier SRStransmission and perform a switched carrier SRS transmission on at leastone candidate carrier of the one or more candidate carriers that isdetermined to be available as a result of performing the uplink LBT onthe one or more candidate carriers. In some embodiments, the UE isfurther adapted to operate according to any of the other embodiments ofthe method of operation of the UE described herein.

In some embodiments, a UE for a wireless system comprises at least onetransceiver, at least one processor, and memory comprising instructionsexecutable by the at least one processor whereby the UE is operable toperform uplink LBT on one or more candidate carriers for switchedcarrier SRS transmission and perform a switched carrier SRS transmissionon at least one candidate carrier of the one or more candidate carriersthat is determined to be available as a result of performing the uplinkLBT on the one or more candidate carriers.

In some embodiments, a UE for a wireless system comprises an uplink LBTmodule operable to perform uplink LBT on one or more candidate carriersfor switched carrier SRS transmission and a switched carrier SRStransmission module operable to perform a switched carrier SRStransmission on at least one candidate carrier of the one or morecandidate carriers that is determined to be available as a result ofperforming the uplink LBT on the one or more candidate carriers.

In some embodiments, a method of operation of a UE in a wireless systemcomprises receiving an indication of an SRS switching opportunity from anetwork node and performing one or more switched carrier SRStransmissions in accordance with the indication of the SRS switchingopportunity. In some embodiments, the indication of the SRS switchingopportunity is any one or any combination of: an indication of aspecific subframe in which to attempt SRS Carrier Based Switching (SCBS)on one or more switched carriers, a number of contiguous SRS symbolsthat are to be transmitted with switching is performed in a downlinkpartial ending subframe, a set of carriers on which to attempt SRSswitching, an indication of one or more uplink interlaces on which theswitched SRS transmissions are to be transmitted, a contention-window orClear Channel Assessment (CCA) duration to be used for uplink LBT forswitched SRS transmissions, an indication of whether uplink LBT can beskipped prior to SRS transmission on a switched carrier, and anindication of whether SRS switching is to be triggered to deferred to anext periodic opportunity.

In some embodiments, a UE for a wireless system is adapted to receive anindication of an SRS switching opportunity from a network node andperform one or more switched carrier SRS transmissions in accordancewith the indication of the SRS switching opportunity.

In some embodiments, a UE for a wireless system comprises at least onetransceiver, at least one processor, and memory comprising instructionsexecutable by the at least one processor whereby the UE is operable toreceive an indication of an SRS switching opportunity from a networknode and perform one or more switched carrier SRS transmissions inaccordance with the indication of the SRS switching opportunity.

In some embodiments, a UE for a wireless system comprises a receivingmodule operable to receive an indication of an SRS switching opportunityfrom a network node and a performing module operable to perform one ormore switched carrier SRS transmissions in accordance with theindication of the SRS switching opportunity.

BRIEF DESCRIPTION OF THE DRAWINGS

The accompanying drawing figures incorporated in and forming a part ofthis specification illustrate several aspects of the disclosure, andtogether with the description serve to explain the principles of thedisclosure.

FIG. 1 illustrates a time-frequency grid representing the basic LongTerm Evolution (LTE) downlink physical resource;

FIG. 2 illustrates an LTE downlink frame structure;

FIG. 3 illustrates an example uplink frame structure in LTE;

FIG. 4 illustrates License Assisted Access (LAA) to unlicensed spectrumusing LTE;

FIG. 5 illustrates one example of an interlace for uplink transmission;

FIG. 6 illustrates an example of interlace-based Sounding ReferenceSignal (SRS) in MulteFire;

FIG. 7 illustrates one example of a wireless system in which embodimentsof the present disclosure may be implemented;

FIG. 8 illustrates a User Equipment device (UE) that is configured toperform Physical Uplink Shared Channel (PUSCH) transmissions onComponent Carrier 1 (CC1) according to one embodiment of the presentdisclosure;

FIG. 9 illustrates one example of the first embodiment of the presentdisclosure;

FIG. 10 illustrates one example of the second aspect of the firstembodiment of the present disclosure;

FIG. 11 illustrates one example of multiplexing of switched SRStransmissions from different groups of UEs according to some embodimentsof the present disclosure;

FIG. 12 illustrates mechanisms for indication of an SRS switchingopportunity by the base station according to one embodiment of thepresent disclosure;

FIGS. 13 through 15 illustrate various embodiments of a base station;and

FIGS. 16 and 17 illustrate various embodiments of a UE.

DETAILED DESCRIPTION

The embodiments set forth below represent information to enable thoseskilled in the art to practice the embodiments and illustrate the bestmode of practicing the embodiments. Upon reading the followingdescription in light of the accompanying drawing figures, those skilledin the art will understand the concepts of the disclosure and willrecognize applications of these concepts not particularly addressedherein. It should be understood that these concepts and applicationsfall within the scope of the disclosure and the accompanying claims.

Radio Node: As used herein, a “radio node” is either a radio access nodeor a wireless device.

Radio Access Node: As used herein, a “radio access node” is any node ina radio access network of a cellular communications network thatoperates to wirelessly transmit and/or receive signals. Some examples ofa radio access node include, but are not limited to, a base station(e.g., an enhanced or evolved Node B (eNB) in a Third GenerationPartnership Project (3GPP) Long Term Evolution (LTE) network), ahigh-power or macro base station, a low-power base station (e.g., amicro base station, a pico base station, a home eNB, or the like), and arelay node.

Core Network Node: As used herein, a “core network node” is any type ofnode in a Core Network (CN). Some examples of a core network nodeinclude, e.g., a Mobility Management Entity (MME), a Packet Data Network(PDN) Gateway (P-GW), a Service Capability Exposure Function (SCEF), orthe like.

Wireless Device or User Equipment device (UE): As used herein, a“wireless device” or “UE” is any type of device that has access to(i.e., is served by) a wireless network (e.g., a cellular communicationsnetwork) by wirelessly transmitting and/or receiving signals to a radioaccess node(s). Some examples of a wireless device include, but are notlimited to, a UE in a 3GPP network and a

Machine Type Communication (MTC) device.

Network Node: As used herein, a “network node” is any node that iseither part of the radio access network or the CN of a cellularcommunications network/system.

Note that the description given herein focuses on a 3GPP cellularcommunications system and, as such, 3GPP LTE terminology or terminologysimilar to 3GPP LTE terminology is oftentimes used. However, theconcepts disclosed herein are not limited to LTE or a 3GPP system.

Note that, in the description herein, reference may be made to the term“cell;” however, particularly with respect to Fifth Generation (5G)concepts, beams may be used instead of cells and, as such, it isimportant to note that the concepts described herein are equallyapplicable to both cells and beams.

In this section, the present disclosure will be illustrated in moredetail by a number of exemplary embodiments. It should be noted thatproposed methods can be applied to different variations of LTE operatingin unlicensed spectrum, such as License Assisted Access (LAA) andMulteFire, as well as to 5G New Radio (NR) in unlicensed spectrum afterappropriate scaling of subcarrier spacing/Transmit Time Interval (TTI)duration compared to LTE.

The present disclosure describes new designs for Sounding ReferenceSignal (SRS) carrier based switching in unlicensed spectrum. Twoseparate cases are considered: legacy SRS as in enhanced LAA (eLAA), andinterlaced SRS as applicable to MulteFire.

Embodiments disclosed herein have the following advantages. SRS CarrierBased Switching (SCBS) is enabled on unlicensed carriers. SRSmultiplexing rules for UEs that are switching SRS to a particularcarrier and UEs that can send Physical Uplink Shared Channel (PUSCH) onthat carrier are defined for both eLAA with legacy SRS and MulteFirewith interlaced SRS. Efficient rules are defined for the indication ofSRS switching opportunities by the LAA/MulteFire eNB.

FIG. 7 illustrates one example of a wireless system 10 in whichembodiments of the present disclosure may be implemented. The wirelesssystem 10 may be a cellular communications network. The wireless system10 includes a base station 12 that operates to serve a number of cells(Cell 1 to Cell N) on a number of different carriers (Carrier 1 toCarrier N, respectively). Some or all of the carriers are in anunlicensed frequency spectrum or some other spectrum that requiresListen-Before-Talk (LBT), or carrier sensing, prior to transmission. Inthis regard, the wireless system 10 may be an LAA system in which atleast one of the carriers is in a licensed spectrum and at least some ofthe other carriers are in an unlicensed spectrum. In other embodiments,the wireless system 10 is a MulteFire or similar system providingstandalone operation in an unlicensed frequency spectrum (i.e., thecarriers are all in the unlicensed spectrum or standalone operation isprovided on at least some of the carriers in an unlicensed spectrum).The base station 12 may be, for example, an eNB in a LTE or LTE-basedcellular communications network.

The base station 12 provides wireless access to a number of UEs 14,which may also be referred to herein as wireless devices. Note thatwhile only one base station 12 is illustrated in this example, the cellsmay alternatively be provided by multiple base stations or radio accessnodes.

The first embodiment focuses on the choice of the carrier to which SRSswitching is performed. In the first aspect, the choice of which carrierto switch to is based on the outcome of uplink LBT performed by the UE14 on a set of one or more candidate switching carriers (i.e., one ormore of the carriers that are candidates for switching SRStransmission), excluding the carriers on which it is currentlyconfigured and scheduled to transmit PUSCH/Physical Uplink ControlChannel (PUCCH).

A first example is shown in FIG. 8 where the UE 14 is configured toperform PUSCH transmissions on Component Carrier 1 (CC1). The UE 14 thenperforms an uplink LBT operation on CC2, which finds the channel to beunoccupied. This is then followed by a switched SRS transmission on CC2,after which the UE 14 switches back to CC1, performs an uplink LBT onCC1, and resumes any PUSCH/PUCCH transmissions that it has beenscheduled for. The uplink LBT operation on CC2 may be of a duration thatis specified by the base station 12 (e.g., eNB) via broadcast SystemInformation (SI) or configured via higher layer signaling. The subframein which the UE 14 attempts uplink LBT on CC2 may be determined based onreading Common Physical Downlink Control Channel (C-PDCCH) informationfrom either the serving cell CC1 or CC2, where the C-PDCCH carriesinformation regarding the upcoming downlink-uplink subframe allocationon one or more carriers.

One example of the first embodiment of the present disclosure isillustrated in FIG. 9. Optional steps are illustrated by dashed lines.Also, unless otherwise explicitly stated or otherwise required, thesteps may be performed in any order. As illustrated, the base station 12optionally configures the UE 14 to perform shared channel uplinktransmission (e.g., PUSCH transmission) on a first carrier (step 100).Optionally, the UE 14 begins uplink transmission on the first carrieraccording to the configuration of step 100 (step 102). The UE 14performs uplink LBT on a candidate carrier for switched carrier SRStransmission (step 104). The candidate carrier is a carrier other thanthe first carrier configured for the UE 14 and on which the UE 14 isscheduled to transmit. In this example, the uplink LBT proceduredetermines that the candidate carrier is available and, as such, the UE14 performs a switched carrier SRS transmission on the candidate carrier(step 106). In other words, the UE 14 suspends uplink transmission onthe first carrier, switches to the candidate carrier, and transmits SRSon the candidate carrier. The base station 12 receives and processes theswitched carrier SRS transmission (step 108). The UE 14 performs uplinkLBT on the first carrier (step 110). Upon determining that the firstcarrier is available, the UE 14 resumes uplink transmission on the firstcarrier (step 112).

In a second aspect, the uplink LBT on candidate switching carriers isperformed together with uplink LBT on carriers for which the UE 14 hasreceived uplink PUSCH/PUCCH grants. For multicarrier uplink LBT, the LBTmechanism can be based on either ‘Type A’ or ‘Type B.’ In Type A,independent random backoff is performed on each candidate CC, while inType B, a full random backoff with multiple Clear Channel Assessment(CCA) slots is performed on a specific carrier, while a quick CCA check(e.g., of duration 25 microseconds (μs)) is performed on all othercarriers before the start of transmission on the random backoff carrier.In both types, uplink transmission is performed only on those carrierswhich are deemed to be unoccupied.

Let S1 denote the set of candidate carriers for SRS switching, and S2denote the set of carriers for which the UE 14 has received uplinktransmission grants, S1 and S2 being mutually exclusive (i.e., S1 and S2are disjoint sets). The set S1 may be indicated by the serving cell. Inthe above aspect, the UE 14 performs LBT on both sets S1 and S2simultaneously. If the LBT mechanism is based on Type B, then the UE 14performs a full random backoff on one of the carriers in set S2, andutilizes a quick CCA check on all other carriers in S1 and

S2. If the LBT mechanism is based on Type A, then the UE 14 utilizes thesame Contention Window (CW) for carriers in S1 as the CW used forcarriers in S2, where the CW may have been indicated in the uplinkgrant(s) for S2 by the eNB.

One example of the second aspect of the first embodiment of the presentdisclosure is illustrated in FIG. 10. Optional steps are illustrated bydashed lines. Also, unless otherwise explicitly stated or otherwiserequired, the steps may be performed in any order. As illustrated, thebase station 12 optionally transmits a grant to the UE 14 for uplinktransmission on a first set of carriers (S1) (step 200). The grant is,in some embodiments, a grant for a PUSCH transmission and/or a PUCCHtransmission. The first set of carriers (S1) includes one or morecarriers.

The UE 14 performs multi-carrier uplink LBT on the first set of carriers(S1) and a second set of candidate carriers (S2) for switched carrierSRS transmission (step 202). The second set of candidate carriers (S2)includes one or more carriers. Further, the first and second sets ofcarriers (S1 and S2) are mutually exclusive. Multi-carrier uplink LBT isa procedure by which the UE 14 simultaneously performs uplink LBT onmultiple carriers, where in step 202 these multiple carriers include thecarriers in the first and second sets of carriers (S1 and S2).Optionally, the UE 14 selects one or more of the candidate carriers inthe second set (S2) that are available, as determined by themulti-carrier uplink

LBT procedure, for switched carrier SRS transmission (step 204). The UE14 performs switched carrier SRS transmission on the (selected)candidate carrier(s) (step 206). Alternatively, the UE 14 may performswitched carrier SRS transmission on all available carriers in thesecond set (S2). In some embodiments, the UE 14 suspends uplinktransmission on at least one and possibly all of the carriers in thefirst set of carriers (S1) in order to transmit SRS on the (selected)candidate carriers from the second set (S2). The base station 12receives and processes the uplink switched carrier SRS transmission(step 208).

In some embodiments, multiplexing of switched SRS transmissions isprovided. In these embodiments, switched SRS transmissions from UEs thatare switching from another CC are multiplexed with ongoing transmissionsfrom UEs configured to use the same CC for their uplink transmissions.As a non-limiting example, consider two groups of UEs, D1 and D2, and anuplink CC, CC1. UEs in group D1 switch from CCs other than CC1 totransmit SRS on CC1, i.e., these UEs perform SCBS. UEs in group D2 areconfigured for uplink PUSCH, PUCCH, and SRS transmissions on CC1, i.e.,they are not performing SCBS.

The first aspect relates to switched SRS multiplexing for eLAAtransmissions. In one non-limiting example, the switched SRStransmissions from group D1 UEs are multiplexed with SRS transmissionsfrom group D2 UEs in symbol 13 of the same subframe (uplink or downlinkpartial ending). The group D2 UEs are configured to transmit SRS withoutPUSCH, such that a gap is available prior to symbol 13 for uplink LBT byboth group D1 and D2 UEs. The switched group D1 SRSs and regular groupD2 SRSs are assigned different combs and/or cyclic shifts in order tomultiplex them in the same symbol location.

The second aspect relates to switched SRS multiplexing for MulteFireCCs. When the SRS multiplexing is performed with multiple SRS symbols inthe downlink partial ending subframe (Uplink Pilot Time Slot (UpPTS))region of the switched carrier, time-domain Orthogonal Cover Codes(OCCs) may be used to pack group D1 and D2 UEs on the same uplinkinterlace. For four symbols, this OCC can be defined as a set of fourconfigurations:

w ∈ {[1, 1, 1, 1], [1, 1, −1, −1], [1, −1, 1, −1], [1, −1, −1, 1]}.

In addition to time-domain OCC, intra-symbol frequency-domain OCC andcyclic shifts may also be used to multiplex group D1 and D2 UEs on thesame uplink interlace. For a single SRS symbol transmission in an uplinksubframe, intra-symbol frequency-domain OCC and cyclic shifts may alsobe used to multiplex group D1 and D2 UEs on the same uplink interlace.Alternatively, group D1 and group D2 UEs may be assigned differentuplink interlaces. This way, D2 UEs may transmit any uplinkchannel/reference signal such as PUSCH, PUCCH, or SRS in the samesubframe as the switched SRS transmissions from group D1 UEs withoutmutual interference.

FIG. 11 illustrates one example of multiplexing of switched SRStransmissions from different groups of UEs 14 according to someembodiments of the present disclosure. Optional steps are illustrated bydashed lines. Also, unless otherwise explicitly stated or otherwiserequired, the steps may be performed in any order. As illustrated, thebase station 12 optionally configures a first group of UEs 14 with anSRS transmission multiplexing configuration for the first group of UEs14 (group 1 or D1) (step 300) and optionally configures a second groupof UEs 14 with an SRS transmission multiplexing configuration for thesecond group of UEs 14 (group 2 or D2) (step 302), as described above.

The UE(s) 14 in the first group perform uplink LBT on one or morecandidate carrier(s) for switched carrier SRS transmission (step 304).In particular, the UE(s) 14 may perform single carrier uplink LBT (i.e.,uplink LBT on a single carrier) or multi-carrier uplink LBT, e.g., asdescribed above. Optionally, the UE(s) 14 selects one (or potentiallymultiple) candidate carrier(s) that is(are) available for switchedcarrier SRS transmission (step 306). The UE(s) 14 perform switchedcarrier SRS transmission on the (selected) candidate carrier(s)determined to be available using the SRS transmission multiplexingconfiguration for group 1 (step 308). UE(s) 14 in the second groupmultiplex their SRS transmission on the same carrier(s) using the SRStransmission configuration for group 2 (step 310). In other words, usingthe different SRS transmission configurations, the UEs 14 in the firstand second groups are able to multiplex their SRS transmissions on thesame carrier(s). The base station 12 receives and processes the SRStransmissions from the UEs 14 in the first and second groups of UEs(step 312).

In some embodiments, mechanisms for indication of SRS switchingopportunity by the base station 12 (e.g., eNB) are provided. Thefollowing are various aspects and parameters that may be signaled by thebase station 12 either dynamically on the C-PDCCH or Physical HybridAutomatic Repeat Request Indicator Channel (PHICH), or via higher-layersignaling, for SCBS operation on unlicensed carriers:

-   -   The indication of which specific subframe to attempt SCBS on one        or more switched carriers is broadcast dynamically, e.g., in the        C-PDCCH sent on downlink subframes in LAA and MulteFire (as also        described above).    -   The number of contiguous SRS symbols that shall be transmitted        (e.g., 1, 2, 3, or 4 symbols) when switching is performed on        MulteFire CCs in a downlink partial ending subframe.    -   The set of CCs on which to attempt SRS switching.    -   The uplink interlace(s) on which the switched SRSs shall be        transmitted.    -   The CW or CCA duration to be used for uplink LBT for switched        SRSs.    -   If uplink LBT can be skipped prior to SRS transmission on the        switched carrier.    -   If SRS switching should be triggered or deferred to the next        periodic opportunity.

FIG. 12 illustrates one example of this embodiment. As illustrated, thebase station 12 transmits an indication of SRS switching opportunity tothe UE 14 (step 400). The indication may be any one or any combinationof the examples given above. The UE 14 performs switched carrier SRStransmission(s) according to the received indication (step 402).

The present disclosure describes how to implement SRS carrier basedswitching on carriers in unlicensed spectrum. Solutions for both eLAAand MulteFire are presented.

FIG. 13 is a schematic block diagram of the base station 12 according tosome embodiments of the present disclosure. As illustrated, the basestation 12 includes a control system 16 that includes one or moreprocessors 18 (e.g., Central Processing Units (CPUs), ApplicationSpecific Integrated Circuits (ASICs), Field Programmable Gate Arrays(FPGAs), and/or the like), memory 20, and a network interface 22. Inaddition, the base station 12 includes one or more radio units 24 thateach includes one or more transmitters 26 and one or more receivers 28coupled to one or more antennas 30. In some embodiments, the radiounit(s) 24 is external to the control system 16 and connected to thecontrol system 16 via, e.g., a wired connection (e.g., an opticalcable). However, in some other embodiments, the radio unit(s) 24 andpotentially the antenna(s) 30 are integrated together with the controlsystem 16. The one or more processors 18 operate to provide one or morefunctions of a base station 12 as described herein. In some embodiments,the function(s) are implemented in software that is stored, e.g., in thememory 20 and executed by the one or more processors 18.

FIG. 14 is a schematic block diagram that illustrates a virtualizedembodiment of the base station 12 according to some embodiments of thepresent disclosure. This discussion is equally applicable to other typesof network nodes. Further, other types of network nodes may have similarvirtualized architectures.

As used herein, a “virtualized” base station 12 is an implementation ofthe base station 12 in which at least a portion of the functionality ofthe base station 12 is implemented as a virtual component(s) (e.g., viaa virtual machine(s) executing on a physical processing node(s) in anetwork(s)). As illustrated, in this example, the base station 12includes the control system 16 (optional) that includes the one or moreprocessors 18 (e.g., CPUs, ASICs, FPGAs, and/or the like), the memory20, and the network interface 22 and the one or more radio units 24 thateach includes the one or more transmitters 26 and the one or morereceivers 28 coupled to the one or more antennas 30, as described above.The control system 16 is connected to the radio unit(s) 24 via, forexample, an optical cable or the like. The control system 16 isconnected to one or more processing nodes 32 coupled to or included aspart of a network(s) 34 via the network interface 22. Each processingnode 32 includes one or more processors 36 (e.g., CPUs, ASICs, FPGAs,and/or the like), memory 38, and a network interface 40.

In this example, functions 42 of the base station 12 described hereinare implemented at the one or more processing nodes 32 or distributedacross the control system 16 and the one or more processing nodes 32 inany desired manner. In some particular embodiments, some or all of thefunctions 42 of the base station 12 described herein are implemented asvirtual components executed by one or more virtual machines implementedin a virtual environment(s) hosted by the processing node(s) 32. As willbe appreciated by one of ordinary skill in the art, additional signalingor communication between the processing node(s) 32 and the controlsystem 16 is used in order to carry out at least some of the desiredfunctions 42. Notably, in some embodiments, the control system 16 maynot be included, in which case the radio unit(s) 24 communicate directlywith the processing node(s) 32 via an appropriate network interface(s).

In some embodiments, a computer program including instructions which,when executed by at least one processor, causes the at least oneprocessor to carry out the functionality of a base station 12 or a node(e.g., a processing node 32) implementing one or more of the functions42 of the base station 12 in a virtual environment according to any ofthe embodiments described herein is provided. In some embodiments, acarrier comprising the aforementioned computer program product isprovided. The carrier is one of an electronic signal, an optical signal,a radio signal, or a computer readable storage medium (e.g., anon-transitory computer readable medium such as memory).

FIG. 15 is a schematic block diagram of the base station 12 according tosome other embodiments of the present disclosure. The base station 12includes one or more modules 44, each of which is implemented insoftware. The module(s) 44 provide the functionality of the base station12 described herein. This discussion is equally applicable to theprocessing node 32 of FIG. 14 where the modules 44 may be implemented atone of the processing nodes 32 or distributed across multiple processingnodes 32 and/or distributed across the processing node(s) 32 and thecontrol system 16.

FIG. 16 is a schematic block diagram of a UE 14 according to someembodiments of the present disclosure. As illustrated, the UE 14includes one or more processors 46 (e.g., CPUs, ASICs, FPGAs, and/or thelike), memory 48, and one or more transceivers 50 each including one ormore transmitters 52 and one or more receivers 54 coupled to one or moreantennas 56. In some embodiments, the functionality of the UE 14described above may be fully or partially implemented in software thatis, e.g., stored in the memory 48 and executed by the processor(s) 46.

In some embodiments, a computer program including instructions which,when executed by at least one processor, causes the at least oneprocessor to carry out the functionality of the UE 14 according to anyof the embodiments described herein is provided. In some embodiments, acarrier comprising the aforementioned computer program product isprovided. The carrier is one of an electronic signal, an optical signal,a radio signal, or a computer readable storage medium (e.g., anon-transitory computer readable medium such as memory).

FIG. 17 is a schematic block diagram of the UE 14 according to someother embodiments of the present disclosure. The UE 14 includes one ormore modules 58, each of which is implemented in software. The module(s)58 provide the functionality of the UE 14 described herein.

While not being limited thereto, some example embodiments of the presentdisclosure are provided below.

FIRST EMBODIMENT

A method of operation of a UE (14) in a wireless system (10) comprisesperforming (104, 202, 304) uplink LBT on one or more candidate carriersfor switched carrier SRS transmission, and performing (106, 206, 308) aswitched carrier SRS transmission on at least one candidate carrier ofthe one or more candidate carriers that is determined to be available asa result of performing (104, 202, 304) the uplink LBT on the one or morecandidate carriers.

SECOND EMBODIMENT

The first embodiment wherein the one or more candidate carriers arecarriers other than carriers configured for the UE (14) and on which theUE (14) is scheduled to transmit.

THIRD EMBODIMENT

The first or second embodiment wherein performing (104, 202, 304) theuplink LBT on the one or more candidate carriers for switched carrierSRS transmission comprises performing (104) the uplink LBT on acandidate carrier, a result of the uplink LBT being that the candidatecarrier is available; and performing (106, 206, 308) the switchedcarrier SRS transmission on the at least one candidate carrier comprisesperforming (106) the switched carrier SRS transmission on the candidatecarrier in response to the result of the uplink LBT being that thecandidate carrier is available.

FOURTH EMBODIMENT

The first or second embodiment wherein performing (104, 202, 304) theuplink LBT on the one or more candidate carriers for switched carrierSRS transmission comprises performing (202) multi-carrier uplink LBT ona first set of carriers for which the UE (14) is granted uplinktransmission and a second set of carriers comprising the one or morecandidate carriers; and wherein the at least one candidate carrier onwhich the UE (14) performs switched carrier SRS transmission comprisesat least one candidate carrier from the one or more candidate carriersin the second set of carriers.

FIFTH EMBODIMENT

Any one of the first through fourth embodiments wherein performing (106,206, 308) the switched carrier SRS transmission on the at least onecandidate carrier comprises performing (308) the switched carrier SRStransmission on the at least one candidate carrier in accordance with anSRS transmission multiplexing configuration for a respective group ofUEs (14) such that the switched carrier SRS transmission on the at leastone candidate carrier is multiplexed with SRS transmission of anothergroup of UEs (14) on the same at least one carrier.

SIXTH EMBODIMENT

A UE (14) for a wireless system (10), the UE (14) adapted to performuplink LBT on one or more candidate carriers for switched carrier SRStransmission, and perform a switched carrier SRS transmission on atleast one candidate carrier of the one or more candidate carriers thatis determined to be available as a result of performing the uplink LBTon the one or more candidate carriers.

SEVENTH EMBODIMENT

The sixth embodiment wherein the UE (14) is further adapted to operateaccording to the method of any one of the second through fifthembodiments.

EIGHTH EMBODIMENT

A UE (14) for a wireless system (10) comprising at least one transceiver(50), at least one processor (46), and memory (48) comprisinginstructions executable by the at least one processor (46) whereby theUE (14) is operable to perform uplink LBT on one or more candidatecarriers for switched carrier SRS transmission and perform a switchedcarrier SRS transmission on at least one candidate carrier of the one ormore candidate carriers that is determined to be available as a resultof performing the uplink LBT on the one or more candidate carriers.

NINTH EMBODIMENT

A UE (14) for a wireless system (10) comprising an uplink LBT module(58) operable to perform uplink LBT on one or more candidate carriersfor switched carrier SRS transmission and a switched carrier SRStransmission module (58) operable to perform a switched carrier SRStransmission on at least one candidate carrier of the one or morecandidate carriers that is determined to be available as a result ofperforming the uplink LBT on the one or more candidate carriers.

The following acronyms are used throughout this disclosure.

-   -   μs Microsecond    -   3GPP Third Generation Partnership Project    -   5G Fifth Generation    -   ASIC Application Specific Integrated Circuit    -   CC Component Carrier    -   CCA Clear Channel Assessment    -   CN Core Network    -   C-PDCCH Common Physical Downlink Control Channel    -   CPU Central Processing Unit    -   CW Contention Window    -   dBm Decibel-Milliwatt    -   B-IFDMA Block-Interleaved Frequency Division Multiple Access    -   DFT Discrete Fourier Transform    -   DMRS Demodulation Reference Signal    -   eLAA Enhanced License Assisted Access    -   eNB Enhanced or Evolved Node B    -   FDMA Frequency Division Multiple Access    -   FPGA Field Programmable Gate Array    -   GHz Gigahertz    -   LAA License Assisted Access    -   LBT Listen-Before-Talk    -   LTE Long Term Evolution    -   LTE-U Long Term Evolution Unlicensed    -   MHz Megahertz    -   MME Mobility Management Entity    -   ms Millisecond    -   MTC Machine Type Communication    -   NR New Radio    -   OCC Orthogonal Cover Code    -   OFDM Orthogonal Frequency Division Multiplexing    -   PCell Primary Cell    -   PDN Packet Data Network    -   PDSCH Physical Downlink Shared Channel    -   P-GW Packet Data Network Gateway    -   PHICH Physical Hybrid Automatic Repeat Request Indicator Channel    -   PRACH Physical Random Access Channel    -   PRB Physical Resource Block    -   PSD Power Spectral Density    -   PUCCH Physical Uplink Control Channel    -   PUSCH Physical Uplink Shared Channel    -   RB Resource Block    -   RE Resource Element    -   Rel Release    -   SCBS Sounding Reference Signal Carrier Based Switching    -   SCell Secondary Cell    -   SCEF Service Capability Exposure Function    -   SC-FDMA Single-Carrier Frequency Division Multiple Access    -   SI System Information    -   sPUCCH Short Physical Uplink Control Channel    -   SRS Sounding Reference Signal    -   TDD Time Division Duplexing    -   TTI Transmit Time Interval    -   UE User Equipment    -   UpPTS Uplink Pilot Time Slot    -   WLAN Wireless Local Area Network

Those skilled in the art will recognize improvements and modificationsto the embodiments of the present disclosure. All such improvements andmodifications are considered within the scope of the concepts disclosedherein and the claims that follow.

What is claimed is:
 1. A method of operation of a User Equipment device,UE, in a wireless system, comprising: prior to performing uplinkListen-Before-Talk, LBT, on one or more candidate carriers, starting anuplink transmission on one or more carriers for which the UE is grantedthe uplink transmission; suspending the uplink transmission on the oneor more carriers for which the UE is granted the uplink transmission;performing the uplink LBT on the one or more candidate carriers forswitched carrier Sounding Reference Signal, SRS, transmission, whereinthe one or more candidate carriers are different from the one or morecarriers for which the UE is granted the uplink transmission; andperforming a switched carrier SRS transmission on at least one candidatecarrier of the one or more candidate carriers that is determined to beavailable as a result of performing the uplink LBT on the one or morecandidate carriers.
 2. The method of claim 1 wherein the one or morecandidate carriers are carriers other than carriers configured for theUE and on which the UE is scheduled to transmit.
 3. The method of claim1 wherein: performing the uplink LBT on the one or more candidatecarriers for switched carrier SRS transmission comprises performing theuplink LBT on a candidate carrier, a result of the uplink LBT being thatthe candidate carrier is available; and performing the switched carrierSRS transmission on the at least one candidate carrier comprisesperforming the switched carrier SRS transmission on the candidatecarrier in response to the result of the uplink LBT being that thecandidate carrier is available.
 4. The method of claim 3 wherein: priorto performing the uplink LBT on the candidate carrier, starting theuplink transmission comprises starting the uplink transmission on afirst carrier, the first carrier being different than the candidatecarrier; and the method further comprises, after performing the switchedcarrier SRS transmission on the candidate carrier: performing uplink LBTon the first carrier, a result of the uplink LBT on the first carrierbeing that the first carrier is available; and resuming the uplinktransmission on the first carrier upon the result of the uplink LBT onthe first carrier being that the first carrier is available.
 5. Themethod of claim 1 wherein: performing the uplink LBT on the one or morecandidate carriers for switched carrier SRS transmission comprisesperforming multi-carrier uplink LBT on a first set of carriers for whichthe UE is granted the uplink transmission and a second set of carrierscomprising the one or more candidate carriers, the first set of carriersand the second set of carriers being disjoint sets; and wherein the atleast one candidate carrier on which the UE performs switched carrierSRS transmission comprises at least one candidate carrier from the oneor more candidate carriers in the second set of carriers.
 6. The methodof claim 5 wherein performing multi-carrier uplink LBT on the first setof carriers for which the UE is granted the uplink transmission and thesecond set of carriers comprising the one or more candidate carrierscomprises suspending the uplink transmission on at least one of thecarriers in the first set of carriers prior to transmitting SRS on theat least one candidate carrier.
 7. The method of claim 5 whereinperforming multi-carrier uplink LBT on the first set of carriers forwhich the UE is granted the uplink transmission and the second set ofcarriers comprising the one or more candidate carriers comprisessuspending the uplink transmission on all of the carriers in the firstset of carriers prior to transmitting SRS on the at least one candidatecarrier.
 8. The method of claim 1 wherein performing the switchedcarrier SRS transmission on the at least one candidate carrier comprisesperforming the switched carrier SRS transmission on the at least onecandidate carrier in accordance with an SRS transmission multiplexingconfiguration for a respective group of UEs such that the switchedcarrier SRS transmission on the at least one candidate carrier ismultiplexed with uplink transmissions of another group of UEs on thesame at least one candidate carrier.
 9. The method of claim 1 whereinperforming the switched carrier SRS transmission on the at least onecandidate carrier comprises performing the switched carrier SRStransmission on the at least one candidate carrier in accordance with anSRS transmission multiplexing configuration for a respective group ofUEs such that the switched carrier SRS transmission on the at least onecandidate carrier is multiplexed with SRS transmission of another groupof UEs on the same at least one candidate carrier.
 10. The method ofclaim 9 wherein the switched carrier SRS transmissions of the respectivegroup of UEs are multiplexed with the SRS transmissions of another groupof UEs in a particular symbol of a same subframe on the same at leastone candidate carrier.
 11. The method of claim 9 wherein the switchedcarrier SRS transmissions of the respective group of UEs are multiplexedwith the SRS transmissions of another group of UEs using differenttime-domain Orthogonal Cover Codes, OCCs.
 12. The method of claim 9wherein the switched carrier SRS transmissions of the respective groupof UEs are multiplexed with the SRS transmissions of another group ofUEs using different time-domain Orthogonal Cover Codes, OCCs, anddifferent intra-symbol frequency-domain OCCs and cyclic shifts.
 13. AUser Equipment device, UE, for a wireless system, comprising: at leastone transceiver; at least one processor; and memory comprisinginstructions executable by the at least one processor whereby the UE isoperable to: prior to performing uplink Listen-Before-Talk, LBT, on oneor more candidate carriers, start an uplink transmission on one or morecarriers for which the UE is granted the uplink transmission; suspendthe uplink transmission on the one or more carriers for which the UE isgranted the uplink transmission; perform the uplink LBT on the one ormore candidate carriers for switched carrier Sounding Reference Signal,SRS, transmission, wherein the one or more candidate carriers aredifferent from the one or more carriers for which the UE is granted theuplink transmission; and perform a switched carrier SRS transmission onat least one candidate carrier of the one or more candidate carriersthat is determined to be available as a result of performing the uplinkLBT on the one or more candidate carriers.
 14. The UE of claim 13wherein the one or more candidate carriers are carriers other thancarriers configured for the UE and on which the UE is scheduled totransmit.
 15. The UE of claim 13 wherein: the one or more candidatecarriers comprise a candidate carrier; and the UE is further operableto: perform the uplink LBT on the candidate carrier, a result of theuplink LBT being that the candidate carrier is available; and performthe switched carrier SRS transmission on the candidate carrier inresponse to the result of the uplink LBT being that the candidatecarrier is available.
 16. The UE of claim 15 wherein: the one or morecarriers for which the UE is granted the uplink transmission comprise afirst carrier, the first carrier being different than the candidatecarrier; and the UE is further operable to: prior to performing theuplink LBT on the candidate carrier, start the uplink transmission onthe first carrier; and after performing the switched carrier SRStransmission on the candidate carrier: perform uplink LBT on the firstcarrier, a result of the uplink LBT on the first carrier being that thefirst carrier is available; and resume the uplink transmission on thefirst carrier upon the result of the uplink LBT on the first carrierbeing that the first carrier is available.
 17. The UE of claim 13wherein: in order to perform the uplink LBT on the one or more candidatecarriers for switched carrier SRS transmission, the UE is furtheroperable to perform multi-carrier uplink LBT on a first set of carriersfor which the UE is granted the uplink transmission and a second set ofcarriers comprising the one or more candidate carriers, the first set ofcarriers and the second set of carriers being disjoint sets; and whereinthe at least one candidate carrier on which the UE performs switchedcarrier SRS transmission comprises at least one candidate carrier fromthe one or more candidate carriers in the second set of carriers. 18.The UE of claim 17 wherein, in order to perform multi-carrier uplink LBTon the first set of carriers for which the UE is granted the uplinktransmission and the second set of carriers comprising the one or morecandidate carriers, the UE is further operable to suspend the uplinktransmission on at least one of the carriers in the first set ofcarriers prior to transmitting SRS on the at least one candidatecarrier.
 19. The UE of claim 17 wherein, in order to performmulti-carrier uplink LBT on the first set of carriers for which the UEis granted the uplink transmission and the second set of carriers, theUE is further operable to comprise the one or more candidate carrierscomprises suspending the uplink transmission on all of the carriers inthe first set of carriers prior to transmitting SRS on the at least onecandidate carrier.
 20. The UE of claim 13 wherein, in order to performthe switched carrier SRS transmission on the at least one candidatecarrier, the UE is further operable to perform the switched carrier SRStransmission on the at least one candidate carrier in accordance with anSRS transmission multiplexing configuration for a respective group ofUEs such that the switched carrier SRS transmission on the at least onecandidate carrier is multiplexed with uplink transmissions of anothergroup of UEs on the same at least one candidate carrier.
 21. The UE ofclaim 13 wherein, in order to perform the switched carrier SRStransmission on the at least one candidate carrier, the UE is furtheroperable to perform the switched carrier SRS transmission on the atleast one candidate carrier in accordance with an SRS transmissionmultiplexing configuration for a respective group of UEs such that theswitched carrier SRS transmission on the at least one candidate carrieris multiplexed with SRS transmission of another group of UEs on the sameat least one candidate carrier.
 22. The UE of claim 21 wherein theswitched carrier SRS transmissions of the respective group of UEs aremultiplexed with the SRS transmissions of another group of UEs in aparticular symbol of a same subframe on the same at least one candidatecarrier.
 23. The UE of claim 21 wherein the switched carrier SRStransmissions of the respective group of UEs are multiplexed with theSRS transmissions of another group of UEs using different time-domainOrthogonal Cover Codes, OCCs.
 24. The UE of claim 21 wherein theswitched carrier SRS transmissions of the respective group of UEs aremultiplexed with the SRS transmissions of another group of UEs usingdifferent time-domain Orthogonal Cover Codes, OCCs, and differentintra-symbol frequency-domain OCCs and cyclic shifts.